Science fiction is a realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the scientific method.

Arthur C. Clarke – Science fiction is something that could happen – but you usually wouldn’t want it to. Fantasy is something that couldn’t happen – though you often only wish that it could. Attempting to define science fiction is an undertaking almost as difficult, though not so popular, as trying to define pornography… In both pornography and SF, the problem lies in knowing exactly where to draw the line.

Barry N. Malzberg – Science fiction is „that branch of fiction that deals with the possible effects of an altered technology or social system on mankind in an imagined future, an altered present, or an alternative past.

Basil Davenport – Science fiction is fiction based upon some imagined development of science, or upon the extrapolation of a tendency in society.

Brian Stableford – Science fiction is essentially a kind of fiction in which people learn more about how to live in the real world, visiting imaginary worlds unlike our own, in order to investigate by way of pleasurable thought-experiments how things might be done differently.

Brian W. Aldiss – Science fiction is the search for a definition of man and his status in the universe which will stand in our advanced but confused state of knowledge (science), and is characteristically cast in the Gothic or post-Gothic mould. My briefest ever definition of science fiction is „Hubris clobbered by Nemesis”.

Christopher Evans – Perhaps the crispest definition is that science fiction is a literature of ‘what if?’ What if we could travel in time? What if we were living on other planets? What if we made contact with alien races? And so on. The starting point is that the writer supposes things are different from how we know them to be.

Darko R. Suvin – Science fiction in general – through its long history in different contexts – can be defined as a literary genre whose necessary and sufficient conditions are the presence and interaction of estrangement and cognition, and whose main formal device is an imaginative framework alternative to the author’s empirical environment. SF is distinguished by the narrative dominance or hegemony of a fictional „novum” (novelty, innovation) validated by cognitive logic.

David Brin – Many people have tried to define science fiction. I like to call it the literature of exploration and change. While other genres obsess upon so-called eternal verities, SF deals with the possibility that our children may have different problems. They may, indeed, be different than we have been.”

David Ketterer – Philosophically oriented science fiction, extrapolating on what we know in the context of our vaster ignorance, comes up with a startling donnée, or rationale, that puts humanity in a radically new perspective.

David Pringle – Science fiction is a form of fantastic fiction which exploits the imaginative perspectives of modern science.

Dictionary.com – A form of fiction that draws imaginatively on scientific knowledge and speculation in its plot, setting, theme, etc.

Edmund Crispin – A science fiction story is one which presupposes a technology, or an effect of technology, or a disturbance in the natural order, such as humanity, up to the time of writing, has not in actual fact, experienced.

Everett K. Bleiler – Science fiction is not a unitary genre or form, hence cannot be encompassed in a single definition. It is an assemblage of genres and sub-genres that are not intrinsically closely related, but are generally accepted as an area of publication by a marketplace. Science fiction is thus only a commercial term.

Frederik G. Pohl – (when he was editor of Galaxy) Science fiction is a story I can publish in the magazine without having too many readers cancel their subscriptions.

Gary Westfahl – Science fiction is a prose narrative which describes or depicts some aspect or development which does not exist at the time of writing; one significant subgroup of science fiction additionally includes language which either describes scientific fact or explains or reflects the process of scientific thought.

Gregory Benford – SF is a controlled way to think and dream about the future. An integration of the mood and attitude of science (the objective universe) with the fears and hopes that spring from the unconscious. Anything that turns you and your social context, the social you, inside out. Nightmares and visions, always outlined by the barely possible.

Hugo Gernsback – By ‘scientifiction’ I mean the Jules Verne, H. G. Wells and Edgar Allan Poe type of story – a charming romance intermingled with scientific fact and prophetic vision… Not only do these amazing tales make tremendously interesting reading – they are always instructive. They supply knowledge… in a very palatable form… New adventures pictured for us in the scientifiction of today are not at all impossible of realization tomorrow… Many great science stories destined to be of historical interest are still to be written… Posterity will point to them as having blazed a new trail, not only in literature and fiction, but progress as well. Science-fiction… can be defined as: Imaginative extrapolation of true natural phenomena, existing now, or likely to exist in the future.”

Ian S. Menzies – I would define science fiction as a form of literature which crosses the frontiers of knowledge using imagination, intuition or logic to guide it. At times correct scientific or technical detail is demanded by an exacting readership; where the story goes beyond known facts, the deductions must be feasible or at least not in obvious conflict with accepted theories.

Isaac Asimov – Science fiction can be defined as that branch of literature which deals with the reaction of human beings to changes in science and technology. Social science fiction is that branch of literature which is concerned with the impact of scientific advance on human beings. Hard science fiction is stories that feature authentic scientific knowledge and depend upon it for plot development and plot resolution.

J.O. Bailey – A piece of scientific fiction is a narrative of an imaginary invention or discovery in the natural sciences and consequent adventures and experiences… It must be a scientific discovery – something that the author at least rationalizes as possible to science.

Jack Williamson – Science fiction is a specialized type of fantasy, in which the prime assumption usually is a new scientific discovery or invention.

James Blish – Science fantasy is a kind of hybrid in which plausibility is specifically invoked for most of the story, but may be cast aside in patches at the author’s whim and according to no visible system or principle.

James E. Gunn – Science fiction is the branch of literature that deals with the effects of change on people in the real world as it can be projected into the past, the future, or to distant places. It often concerns itself with scientific or technological change, and it usually involves matters whose importance is greater than the individual or the community; often civilization or the race itself is in danger.

Jeff Prucher – Science fiction is a genre (of literature, film, etc.) in which the setting differs from our own world (e.g. by the invention of new technology, through contact with aliens, by having a different history, etc.), and in which the difference is based on extrapolations made from one or more changes or suppositions; hence, such a genre in which the difference is explained (explicitly or implicitly) in scientific or rational, as opposed to supernatural, terms.

Joanna Russ – Science fiction writes about what is neither impossible nor possible; the fact is that, when the question of possibility comes up in science fiction, the author can only reply that nobody knows. We haven’t been there yet. We haven’t discovered that yet. Science fiction hasn’t happened.

John Boyd – … storytelling, usually imaginative as distinct from realistic fiction, which poses the effects of current or extrapolated scientific discoveries, or a single discovery, on the behavior of individuals or society.

John Brunner – As its best, SF is the medium in which our miserable certainty that tomorrow will be different from today in ways we cant predict, can be transmuted to a sense of excitement and anticipation, occasionally evolving into awe. Poised between intransigent skepticism and uncritical credulity, it is par excellence the literature of the open mind.

John W. Campbell – Scientific methodology involves the proposition that a well-constructed theory will not only explain every known phenomenon, but will also predict new and still undiscovered phenomena. Science-fiction tries to do much the same – and write up, in story form, what the results look like when applied not only to machines, but to human society as well. To be science fiction, not fantasy, an honest effort at prophetic extrapolation from the known must be made.”

Judith Merril – Speculative fiction: stories whose objective is to explore, to discover, to learn, by means of projection, extrapolation, analogue, hypothesis-and-paper-experimentation, something about the nature of the universe, of man, or „reality” … I use the term „speculative fiction” here specifically to describe the mode which makes use of the traditional „scientific method” (observation, hypothesis, experiment) to examine some postulated approximation of reality, by introducing a given set of changes – imaginary or inventive – into the common background of „known facts”, creating an environment in which the responses and perceptions of the characters will reveal something about the inventions, the characters, or both.

Kim Stanley Robinson – SF is an historical literature… In every SF narrative, there is an explicit or implicit fictional history that connects the period depicted to our present moment, or to some moment in our past.

Kingsley Amis – Science fiction is that class of prose narrative treating of a situation that could not arise in the world we know, but which is hypothesized on the basis of some innovation in science or technology, or pseudo-science or pseudo-technology, whether human or extra-terrestrial in origin.

Larry Niven – The brightest minds in our field have been trying to find a definition of science fiction for these past seventy years. The short answer is, science fiction stories are given as possible, not necessarily here and now, but somewhere, sometime.

Lester Del Rey – Science fiction is the attempt to deal rationally with alternate possibilities in a manner which will be entertaining.

Margaret Atwood – I define science fiction as fiction in which things happen that are not possible today – that depend, for instance, on advanced space travel, time travel, the discovery of green monsters on other planets or galaxies, or that contain various technologies we have not yet developed.

Mark C. Glassy – The definition of science fiction is like the definition of pornography: you do not know what it is, but you know it when you see it.

Merriam Webster – Fiction dealing principally with the impact of actual or imagined science on society or individuals, or more generally, literary fantasy including a scientific factor as an essential orienting component. Precursors of the genre include Mary Shelley’s Frankenstein (1818), Robert Louis Stevenson’s The Strange Case of Dr. Jekyll and Mr. Hyde (1886), and Jonathan Swift’s Gulliver’s Travels (1726). From its beginnings in the works of Jules Verne and H.G. Wells, it emerged as a self-conscious genre in the pulp magazine Amazing Stories, founded in 1926. It came into its own as serious fiction in the magazine Astounding Science Fiction in the late 1930’s and in works by such writers as Isaac Asimov, Arthur C. Clarke, and Robert Heinlein. A great boom in popularity followed World War II, when numerous writers’ approaches included predictions of future societies on Earth, analyses of the consequences of interstellar travel, and imaginative explorations of intelligent life in other worlds. Much recent fiction has been written in the “cyberpunk” genre, which deals with the effects of computers and artificial intelligence on anarchic future societies. Radio, film, and television have reinforced the popularity of the genre.

Nalo Hopkinson – Literatures that explore the fact that we are toolmakers and users, and are always changing our environment.

Norman Spinrad – There is only one definition of science fiction that seems to make sense: Science fiction is anything published as science fiction.

Northrop Frye – Science fiction is a mode of romance with a strong inherent tendency to myth.

Octavia E. Butler – It doesn’t really mean anything at all, except that if you use science, you should use it correctly, and if you use your imagination to extend it beyond what we already know, you should do that intelligently. Some (SF writers) look for technological solutions and others disparage technological solutions. Some write about the problems that I write about, and others write about other problems. Some think the world will go to hell and others think it will turn into ice cream. You have the same wide variety in science fiction that you have any place.

Ray Bradbury – I define science fiction as the art of the possible. Fantasy is the art of the impossible. Science fiction, again, is the history of ideas, and they’re always ideas that work themselves out and become real and happen in the world. And fantasy comes along and says, „We’re going to break all the laws of physics.”… Most people don’t realize it, but the series of films which have made more money than any other series of films in the history of the universe is the James Bond series. They’re all science fiction, too — romantic, adventurous, frivolous, fantastic science fiction!. Science fiction is the one field that reached out and embraced every sector of the human imagination, every endeavor, every idea, every technological development, and every dream. I called us a nation of Ardent Blasphemers. We ran about measuring not only how things were but how they ought to be… We Americans are better than we hope and worse than we think, which is to say, we are the most paradoxical of all of the paradoxical nations in time. Which is what science fiction is all about. For science fiction runs out with tapes to measure Now against Then against Tomorrow Breakfast. It triangulates mankind amongst these geometrical threads, praising him, warning him. For, above all, science fiction, as far back as Plato trying to figure out a proper society, has always been a fable teacher of morality… There is no large problem in the world this afternoon that is not a science-fictional problem. Science fiction then is the fiction of revolutions. Revolutions in time, space, medicine, travel, and thought… Above all, science fiction is the fiction of warm-blooded human men and women sometimes elevated and sometimes crushed by their machines. So science fiction, we now see, is interested in more than sciences, more than machines. That more is always men and women and children themselves, how they behave, how they hope to behave. Science fiction is apprehensive of future modes of behavior as well as future constructions of metal. Science fiction guesses at sciences before they are sprung out of the brows of thinking men. More, the authors in the field try to guess at machines which are the fruit of these sciences. Then we try to guess at how mankind will react to these machines, how use them, how grow with them, how be destroyed by them. All, all of it fantastic.

Robert A. Heinlein – Science fiction is a realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the scientific method. To make this definition cover all science fiction (instead of ‘almost all’) it is necessary only to strike out the word ‘future’.

Rod Serling – Fantasy is the impossible made probable. Science Fiction is the improbable made possible.

Susan Sontag – Science fiction films are not about science. They are about disaster, which is one of the oldest subjects of art.

The Free Dictionary – A literary or cinematic genre in which fantasy, typically based on speculative scientific discoveries or developments, environmental changes, space travel, or life on other planets, forms part of the plot or background.

Theodore Sturgeon – A science fiction story is a story built around human beings, with a human problem, and a human solution, which would not have happened at all without its scientific content.

Thomas M. Disch – The basic premise of all s-f–that Absolutely Anything Can Happen and Should–has never been so handsomely and hilariously realized as in An Alien Heat.

Tom Shippey – Science fiction is hard to define because it is the literature of change and it changes while you are trying to define it.